Acoustic cleaning of solid parts submersed in a liquid cleaning bath such as, but not limited to, an aqueous cleaning solution, is a known cleaning method. Ultrasonic methods operating at a low acoustic wave frequency of about 20 kHz to 100 kHz may cause violent and random cavitation effects which in turn may produce damage to fragile structures or even erosion of surfaces. In addressing this problem, previous methods have primarily focused on utilizing high frequency ultrasound in the processing and cleaning of semiconductor wafers and other delicate parts. These high frequency systems are single-frequency, continuous wave systems which operate from about 600 kHz to about 2 Mhz, a frequency range which is referred to as “megasonics.” U.S. Pat. No. 3,893,869 discloses a megasonic apparatus comprising means for immersing an article, having a surface to be cleaned, in a container of cleaning fluid. A transducer, adapted to oscillate at a frequency in the range of between 0.2 and 5 MHz, is disposed within the container and positioned so as to produce a beam of ultrasonic energy substantially parallel to the surface to be cleaned. When cleaned, the article is removed from the container and rinsed in a liquid rinse. The article is then dried in clean air at a temperature of between 25° and 300° C. The transducers of available systems involve, as an active element, a piezoelectric ceramic or polymer driven at its resonant frequency by a single frequency continuous-wave generator. This active element converts electrical energy to acoustic energy, and vice versa. The active element may be a piece of polarized material, with electrodes attached to two of its opposite faces, which can be cut in various ways to produce different wave modes. The thickness of the active element is determined by the desired frequency of the transducer. Also relevant is the concept of bandwidth, or range of frequencies, associated with a transducer. The nominal frequency of a transducer is the central frequency and may depend also on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. Megasonic cleaning, which is currently mainly used in the silicon semi-conductor industry, results into a gentler cavitation than ultrasonic cleaning, and may significantly reduce cavitation erosion and the likelihood of surface damage to the product surface being cleaned as compared to low frequency systems. Similar to ultrasonic cleaning, megasonic methods uses a transducer usually made of piezoelectric elements. Cavitation is generally known and defined as the activity of bubbles (e.g., gas bubbles) in a liquid. Such activity includes growth, pulsation and/or collapse of bubbles in a liquid. The pulsation of bubbles is known as stable cavitation, whereas the collapse of bubbles is known as transient cavitation. The occurrence of transient cavitation can release high amounts of energy towards an area surrounding the cavitation. Such energy may be, for example, in the form of heat, shockwaves, etc.
Transient cavitation is applied in a large number of technical fields. For example, in sonochemistry, bubbles collapsing in an ultrasonic field have a catalytic effect on chemical reactions. Also, cavitation is used in medical applications, for example, as a contrast enhancer in ultrasound diagnostics. However, one of the best-known applications of cavitation may be the removal of particles from a surface of a substrate, such as a semiconductor substrate. Megasonic systems are not free from technical problems. A problem of megasonic systems may relates to the nature of high frequency sound waves in a liquid, which travel like a beam within a liquid, and further exhibit higher attenuation than low frequency systems. The beam effect may make it difficult to uniformly fill the cleaning container with the acoustic field.
The sound waves that emanate from an ultrasonic or megasonic transducer originate from multiple points along the surface of the piezoelectric element, and they propagate out from the transducer with a circular wave front. Where the waves interact, there are areas of constructive and destructive interference, the points of constructive interference being often referred to as nodes. Near the face of the transducer, there are extensive fluctuations or nodes and the sound field is very uneven. This is known as the near field or Fresnel zone. The sound field is more uniform away from the transducer in the far field or Fraunhofer zone, where the beam spreads out in a pattern originating from the center of the transducer. But even in the far field it is not a uniform wave front.
U.S. Pat. No. 6,181,051 discloses a system for delivering ultrasound to a liquid, comprising:                one or more ultrasonic transducers, each transducer having an operating frequency within an ultrasound bandwidth; and        an ultrasound generator means for driving the transducers at frequencies within the bandwidth, the generator being amplitude modulated at a modulation frequency and having amplitude modulation frequency sweep means for sweeping the modulation frequency as a function of time, the generator means and transducers being constructed and arranged so as to produce amplitude modulated ultrasound within the liquid.        
Despite some improvements already available from the current megasonic methods and systems delivering energy to a liquid, there is still a need for reducing or eliminating cavitation erosion and surface damage caused to a product or article being immersed in the liquid by providing better control on cavitation and/or on acoustic field uniformity, i.e. by enhancing the use of megasonic energy within the cleaning liquid.